pMDIs are well known in the art of inhalation devices. It is therefore not necessary to describe the construction and operation of a pMDI other than in bare essentials.
A pMDI comprises a canister unit and a housing. The housing is generally tubular, although this is not essential, and generally formed of a plastics material, for instance by moulding. The canister unit comprises an open-ended canister, typically made from a metal such as aluminium. The open end of the canister is sealingly capped by a metering valve assembly. The valve assembly includes a hollow dispensing member or valve stem which projects from the outlet or business end of the canister. The dispensing member is mounted for sliding movement relative to the canister between an extended position, to which the dispensing member is biased by a biasing mechanism in the valve assembly, and a depressed position.
In use, the sealed canister contains a pressurised medicinal aerosol formulation. The formulation comprises the medicament and a fluid propellant, and optionally one or more excipients and/or adjuvants. The medicament is typically in solution or suspension in the formulation. The propellant is typically a CFC-free propellant, suitably a liquid propellant, and may for example be HFA-134a or HFA-227.
Movement of the dispensing member from the extended position to the depressed position results in a metered dose of the aerosol formulation being dispensed from the canister through the dispensing member. Typically, the metering valve assembly is provided with a metering chamber of defined volume. In the extended position of the dispensing member, the content of the canister is placed in fluid communication with the metering chamber through the dispensing member so that the metering chamber is filled with the aerosol formulation. When the dispensing member is depressed, the metering chamber is isolated from the canister inner volume and placed in fluid communication with the external environment through the dispensing member. Thus, the defined volume of the aerosol formulation in the metering chamber is discharged to the external environment via the dispensing member.
Such metering valve assemblies are well known in the art and can be obtained from inter alia Bespak Plc (King's Lynn, Norfolk, United Kingdom) and Valois S.A.S. (Le Neubourg, France).
The housing comprises an internal passageway having an open end. The canister unit is slidable into the internal passageway through the open end with the canister unit being inserted valve assembly first into the internal passageway. A stem block, which receives the dispensing member of the canister when the canister unit is received in the housing in a “rest position”, has a passageway with an inlet end for receiving the dispensing member and an outlet end, which faces a dispensing outlet of the housing, typically a mouthpiece or a nasal nozzle. The stem block holds the dispensing member stationary whereby depression of the canister unit from its rest position further into the housing to an “actuated position” causes the dispensing member to be displaced from the extended position to the depressed position relative to the canister. A metered dose of the aerosol formulation will thereby be dispensed out of the dispensing outlet of the housing via the internal passageway of the stem block.
In use, a patient in need of a metered dose of the medicinal aerosol formulation concurrently inhales on the dispensing outlet and depresses the canister unit from the rest position to the actuated position. The inspiratory airflow produced by the patient entrains the metered dose of the medicinal aerosol formulation into the patient's respiratory tract. This is known in the art as a “breath-coordinated” inhaler because the patient has to coordinate their inhalation with actuation of the inhaler.
Inhalers are commonly provided with a dust cap that covers the dispensing outlet when the inhaler is not in use. The dust cap, when applied, prevents foreign material from entering the housing. This prevents the user from inhaling dust or lint, for example, that might otherwise accumulate in the housing. This is of particular importance where the user suffers from asthma or other respiratory conditions, in which the inhalation of foreign material may cause severe irritation.
Developments to pMDIs have included the provision of actuation indicators or dose counters therefor. Such a dose counter is described in WO-A-9856444, corresponding to U.S. Pat. No. 6,431,168, and WO-A-2004/001664, corresponding to US-A-2006/0096594 of Glaxo Group Limited, all of which are incorporated herein by reference. The pMDI canister unit may comprise the dose counter, which is fixably secured on the valve assembly end of the canister and includes a display which denotes the number of metered doses of the medicament formulation dispensed from, or remaining in, the canister. The display of the dose counter is visible to the patient through a window provided in the housing. The display may be presented by a plurality of indicator wheels rotatably mounted on a common axle, each wheel having numerals from ‘0’ to ‘9’ displayed in series around the circumference.
pMDI devices, however, are susceptible to unintentional actuation, particularly whilst in transit, for example shipment between the manufacturer and distributor. During such transit, such devices and their packaging are often subjected to impacts and sudden movements. Such forces can actuate the pMDI, causing doses of the formulation to be dispensed. When the pMDI includes a dose counter, rough handling in transit can cause the value displayed to the user by the counter to increase or decrease so that it is not consistent with the number of doses that have been dispensed by, or remain in, the pMDI. It is wasteful to dispense unwanted doses of the medicament, and potentially very dangerous for a dose counter to indicate to the user that more doses remain in the canister than are actually present.
Methods of dealing with this problem of unintentional actuation of pMDI devices have previously been described. WO-A-0587299 in the name of Glaxo Group Limited describes a pMDI with a restricting member to prevent unintentional actuation of the inhaler. The restricting member described therein is provided on the inhaler closure and enters the housing through the dispensing outlet when the closure is positioned to close the dispensing outlet. UK Patent Application No. 0505543 and counterpart PCT Patent Application PCT/GB2006/000978, also in the name of Glaxo Group Limited, describe an actuator for a PMDI which comprises a priming mechanism having a support member which includes a stem block. The support member is movable relative to the inhaler housing between a first, inoperative position in which actuation of the inhaler is not possible and a second position in which actuation is possible. U.S. Pat. No. 5,447,150 in the name of Norton Healthcare Limited describes a breath-operated pMDI with a pivotally attached dust cap capable of covering the inhaler mouthpiece. The inhaler may not be actuated when the dust cap is in the closed position.
A multiple-dose DPI with means of preventing unintentional actuation is marketed under the trademark Easyhaler®, the basic inhaler construction being illustrated in WO-A-01/87391 (Orion Corporation). The Easyhaler® inhaler dispenses a powdered medicament when a dosing member is moved, relative to the body of the inhaler, towards a metering drum. This movement causes the drum to rotate, dispensing a single metered dose of the powdered medicament from a powder reservoir at an inhaler mouthpiece for entrainment in the inhalation airflow of a user inhaling thereat, and driving a dose counting mechanism. The inhaler also comprises a small hole through the body of the inhaler, situated above the mouthpiece. A cap is provided, to cover the mouthpiece when not in use, comprising a prong that protrudes through the hole and into the body of the inhaler when the cap is engaged by the mouthpiece. The presence of the prong inside the body of the inhaler restricts the motion of the dosing member in the direction of the drum, preventing the user from dispensing powder by pressing down on the dosing member while the cap is engaged.
An aim of the invention is to provide an inhaler with a novel means of preventing unintentional actuation thereof.